Conventional fuel injection systems utilize a fuel pump to provide fuel to a fuel rail which carries fuel to a plurality of fuel injectors. A pressure regulator is mounted in the fuel flow path so as to maintain the fuel pressure in the rail at approximately 40 psi greater than engine intake manifold vacuum. The pump, typically mounted in the fuel tank, runs at a constant speed and may deliver, for example, 90 liters per hour. When idling, the engine needs only about 3 liters per hour and, therefore, 87 liters per hour must be returned to the fuel tank through a return line. This returned fuel usually has an increased temperature as a result of being routed to the engine and thus frequently evaporates upon reaching the relatively lower pressure and temperature of the fuel tank. The fuel vapor so generated either remains in the tank until vented to atmosphere, which potentially creates environmental problems, or until captured in a vapor storage container, such as a carbon canister, which requires additional manufacturing expense.
In any case, the problems associated with fuel vapor generation in conventional fuel systems have led fuel system designers to develop returnless fuel supply systems, such as that disclosed in U.S. Pat. No. 5,237,975 (Betki et al.). In such a system, fuel rail pressure is controlled for precise fuel mass flow to the injectors at both normal elevated engine temperatures by varying fuel pump speed as a function of assorted variables, including fuel temperature, fuel pressure, engine RPM, and fuel injector pulsewidth. Therefore, no fuel is returned to the fuel tank.
During operation of a vehicle employing a returnless fuel delivery system such as that discussed above, the engine typically cycles through periods of acceleration, intermediate speed operation, deceleration, and idle. To accommodate those cycles, fuel pressure in the fuel rail is varied for proper mass flow. However, during long deceleration periods, pressure within the fuel rail may rise above a level at which effective control of mass flow is possible. For example, pressure within the fuel rail may exceed 70 psi due to high engine temperatures, potentially resulting in reduced fuel economy due to the engine running rich. Excessively high fuel temperatures within the fuel rail may also lead to fuel vaporization resulting in degraded performance due to the engine running lean.
An additional fuel rail pressure problem may occur upon engine start-up if the vehicle is exposed to high ambient temperatures after engine shutoff. In such a circumstance, residual engine heat, along with ambient heat, may cause fuel pressure within the fuel rail to rise above a level effectively controllable by the fuel injectors. High fuel rail pressure may result in fuel leakage through the injectors into the intake manifold, which in turn may cause a rich engine start-up and undesirable exhaust emissions.
One solution to the above-noted fuel rail pressure problems is to provide a pressure relief valve in the fuel line to reduce rail pressure when it exceeds a predetermined value. Some fuel systems have a pressure relief valve connected in a "T" fashion to the fuel line downstream of the pump to and return fuel overage directly to the fuel tank, as disclosed in U.S. Pat. No. 2,881,747 (Gehner). One disadvantage of the "T" configuration is that the relief set pressure of the valve must be set well above system operating pressure since the valve is referenced to tank pressure as opposed to pump output pressure. As a result, the range at which pressure within the fuel rail can be controlled is limited. A second disadvantage of the "T" configuration is that a separate by-pass line and associated fittings are required thus increasing the manufacturing cost and assembly required. The "T" configuration also has the disadvantage of returning fuel overage directly to the fuel tank which may result, particularly under high temperature conditions, in the fuel pump continuously pumping fuel through the pressure valve and back into the fuel tank.
Another fuel line pressure valve is disclosed in U.S. Pat. No. 4,648,369 (Wannenwetsch). That valve serves to prevent unintended fuel injections in a diesel engine by relieving fuel line pressure pulsations caused during operation of a piston-type fuel pump. Such a valve would be inappropriate for a returnless fuel system since its intended purpose is to damp out high-frequency pressure pulsations, not to relieve temperature induced fuel rail pressures. In addition, the valve would be exceedingly difficult to manufacture and assemble, and thus too expensive for large scale automotive production.